1. Technical Field
The present invention relates to a wireless communication system and wireless communication control method, a wireless communication device and wireless communication method, and a computer program for communicating between multiple wireless stations as in a wireless LAN (Local Area Network). More particularly, the present invention relates to a wireless communication system and wireless communication control method, a wireless communication device and wireless communication method, and a computer program for configuring a wireless network by ad-hoc communication without installing a particular device serving as the control station.
More precisely, the present invention relates to a wireless communication device and wireless communication method, a wireless communication device and wireless communication method, and a computer program for configuring a wireless network wherein terminals communicate directly (random access) and asynchronously with each other without installing a particular device serving as the control station, and still more particularly relates to a wireless communication system and wireless communication control method, a wireless communication device and wireless communication method, and a computer program for configuring a wireless network wherein terminals communicate directly (random access) and asynchronously with each other while checking the presence of other terminals.
2. Background Art
Along with increasingly sophisticated computer functions, local area networks (LAN) configured by connecting multiple computers have become a popular method for sharing information such as electronic files and data or peripheral equipment such as printers and for exchanging information by e-mail and data transmission.
In the related art, local area networks (LAN) use optical fibers and coaxial cables or twisted-pair cables and each computer is connected via wire or cable. These wired LAN require installing cables and wiring as well as complicated cable layouts that make it difficult to configure a LAN system. Furthermore, once a LAN system has been configured, equipment and components can only be moved within an area limited by the cable length, making these wired LAN inflexible and inconvenient.
Wireless LAN have become the focus of much attention as network systems that free users from the restrictions of LAN wiring in the related art. These wireless LAN eliminate most of the cable wiring used in work spaces such as in offices and make it relatively easy to move communication terminals such as personal computers (PC).
In recent years, wireless LAN systems have come into increasing demand as their speed becomes faster and cost becomes less expensive. Very recently in particular, personal area networks (PAN) made up of small-scale wireless networks for exchanging information among the multiple pieces of electronic equipment used around people in daily life are under evaluation. The industrial standards IEEE802.11b and IEEE802.11a, for example, are widely known as standards for wireless LAN.
In methods for creating local area networks using wireless technology, one device called the “access point” is installed as a control station in the area, and the network is formed under the overall control of this control station.
In wireless networks installed with this access point, an access control method based on bandwidth reservation is widely employed to transmit information from a communication device. In this method, the bandwidth required to transmit information is reserved at the access point so as to utilize the transmission line without causing conflicts with information transmission by other communication devices. In other words, synchronous wireless communication is performed by installing an access point that permits communication devices in a wireless network to communicate in synchronization with each other.
Another method for configuring a wireless network called “ad-hoc communication” was proposed that allows direct asynchronous communication between terminals. In a small-scale wireless network composed of a relatively small number of clients located near each other, this ad-hoc communication is ideal because all terminals in the network can directly communicate with each other asynchronously without utilizing a particular access point.
In a communication system composed of multiple terminals, access control is essential for preventing communication between terminals from conflicting with each other. Several methods are available for performing access control in a wireless network. These methods can be divided into two groups. One group is channel occupancy methods for example, FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access). The other group is channel sharing methods such as ALOHA and CSMA (Carrier Sense Multiple Access). A method called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is commonly used for access control when performing asynchronous communication in small-scale wireless networks.
In an access control method called CSMA/CD (Carrier Sense Multiple Access with Collision Detection), whether or not conflicts are occurring with information transmissions from other communication devices is detected by receiving a self-issued information signal. This method is mainly used in communication using cable and wires. In wireless communication, on the other hand, it is difficult to receive a self-issued information signal. To avoid conflicts in wireless communication, the CDMA/CA method starts transmitting information after checking that there are no information transmissions from other communication devices. This type of access control allows multiple terminals on the same wireless transmission line to share access and communicate with each other (multiple access)
Another example in the related art of transmission methods for communicating by random access is a method wherein all communication devices connected to one network constantly monitor information being sent through the transmission line and the communication device requested to send information then sends it through the transmission line.
Another further example in the related art of transmission methods for asynchronous communication of the related art is a method wherein each communication device decodes all received signals and only detects information sent addressed to that communication device.
However, wireless communication still has the problem that conflicts occur when multiple communication devices start transmitting information simultaneously. To solve this, a corrective method called RTS/CTS was proposed that transmits information after a connection has been established by sending an RTS (Request to Send) to the receiving destination communication device before transmitting information and also by receiving a CTS (Clear to Send) returned from the information receiving destination.
As stated above, an access point is required for controlling access to perform synchronized wireless communication within a wireless network.
In a wireless communication system having this access point, wireless communication must be performed via this access point during asynchronous communication between the transmitting and receiving communication devices. This creates the problem that transmission line utilization efficiency is reduced by half.
Furthermore, when using a system that reserves a portion of the transmission line, the transmission bandwidth must be reserved after the demand has arisen for asynchronous transmission. In a wireless communication system using time division multiplex access (TDMA), this also creates the problem that information cannot be quickly transmitted.
In such wireless communication systems using time division multiplex access, the terminals must be synchronized with each other even during direct transmission between terminals without via the access point, making it difficult to achieve control.
On the other hand, during asynchronous communication in an ad-hoc wireless network in which terminals can communicate directly without utilizing an access point, the transmitting communication device must check in advance for the receiving communication device.
Furthermore, when transmitting information asynchronously, a redundant preamble signal must also be added to the information so that the receiving communication device can detect that the information transmission has already started.
For example, in UBW (Ultra Wide Band) wireless communication that performs high speed data transmission by diffusing the transmission data into an extremely low frequency bandwidth, there is no carrier such as used for wireless communication in the related art. A preamble signal with even higher redundancy must therefore be added to the beginning of the information transmission so that the receiving terminal can initially synchronize with the information signal by detecting whether the signal is present or not.
When performing access control based on the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) method, the problem occurs that conflicts cannot be detected if multiple communication devices start transmitting information simultaneously.
Asynchronous communication methods of the related art have the fatal flaw that receiving must be performed all the time to capture information that is sent at an unknown time from a communication device in the network. Wireless communication devices must therefore constantly keep receiving in order to perform asynchronous information transmission by wireless communication, making it impossible to reduce power consumption.
In asynchronous receive methods that must decode all the received signals in order to detect information addressed to itself, a large load is imposed on the receive processing. This requires a high speed processor and/or special devices and therefore increases the cost.
When applying asynchronous wireless communication methods of the related art to ultra-high speed transmission lines, a CPU with even greater speed must be used in order to receive multiplexed signals.
When applying the RTS/CTS access control method to asynchronous wireless communication, each terminal must constantly keep receiving in order to detect the short RTS signal. This constant receiving of course makes it difficult to reduce power consumption. Even though standards exist for the RTS/CTS access control method, it is recognized as less effective for use with communication devices.